I was riding a 16" Surly Karate Monkey. I have come to find that, try as I might, I can't make this bike fit me well for the way I ride it. it's just a little too small to ride with a short stem and wide bars, and I would have to put a stem 50mm or so longer to make it work. I want to keep the cockpit set up the same and increase the reach. I have already sold my Karate Monkey frame, so I am in the market for a new frame.

Maybe I am splitting hairs here, but I know from experience how much small changes in geometry can affect a ride. I am 5' 8 1/2" tall with a 32" inseam. I do NOT have the luxury of test riding any bikes.

I have limited options for a replacement frame (I would have to quit my job and get another job at another shop if I want more options, it's that simple), so I am considering two frames with both have a ETT close to 600mm.

rounding these figures to the nearest whole MM:
16" Karate Monkey- 576mm ETT, 591 stack, 395 reach (my old frame, for reference)
18" Karate Monkey- 599mm ETT, 603 stack, 415 reach
17.5" Soma Juice- 595mm ETT, 577 stack, 429 reach
(note that the Juice head tube is 10mm longer)

18" KM ETT is 23mm longer and the reach is 20mm longer.
Juice's ETT is 19mm longer and the reach is 34mm longer.

(a little trig tells me that the distance between the top of the HA and the BB are:
16KM 710mm
18KM 732mm
17.5J 719mm

I don't know if that tells me anything, but there it is.)

I guess the short question is: do I want to get a frame that is appropriately long for me by getting a frame with an appropriately long "reach," or assume that the effective top tube will suit me either way with enough spacers under my stem?